I want to save some money and ditch the $30 unlimited plan I'm on. What plan should I pick? If this isn't a "Your mileage may vary" question, we don't know what is.

First: Check what you've been using over the last six months. You can do that by logging in at the AT&T myWireless Account home page, then clicking on "View Past Data Usage." The default is the last six months.

If you've been using under 200MB on a regular basis, jump on the $15 deal for a $15 savings. More than 200MB? Spring for the $25 2GB plan, and pocket the $5.

Regularly swill more than 2GB a month? Stick to the unlimited plan. (See below for why.)

Pretty simple, really.

Do I have to give up the unlimited plan I have now? No. You're grandfathered in on the current unlimited plan indefinitely, even if you renew your contract or, say, buy a new iPhone this summer.

If I try the $15 200MB plan and it doesn't work out, can I go back to the $30 unlimited? Nope. Once you switch to a new capped plan, you can't return to the unlimited deal.

If I sign up for the 200MB plan, can I later change to the 2GB? Do I have to pay a penalty? Yes on the first, no on the second.

You can bounce between the two capped data plans anytime you want, said AT&T. "There is no early termination fee to move between the new plans," the company's spokeswoman confirmed.

It works the other way, too. Say you pick the 2GB plan, then a month or two later realize you're using just a few score megabytes of data. Boom. You can drop down to the 200MB plan and save $10.

So if I sign up for the 200MB plan and go over the allowance, I pay $15 for another 200GB, or $30 for 400MB total? And 2GB plan customers get another 1GB for just $10? In other words, they pay $35 for 3GB? Did I do the math wrong? You're a regular math genius, you are.

That's how AT&T designed it. So, we asked AT&T to tell us if we read the numbers right, and second, explain why the fees are what they are.

"The overage structure for the two plans was designed to be similar to the data plans themselves," a company spokeswoman said. "In other words, we wanted to let customers receive roughly similar amounts of additional data for approximately the same price. That's why the pricing of the second bucket of data for the [$15] DataPlus plan is the same as the price for the first 200MB. With the [$25] DataPro plan, an additional 2GB will cost slightly less than the first 2GB contained in the plan."